Hello
So let's go trough this step by step:
Note that I mostly refer to gaming, but below information also applies to photoshop, unless stated otherwise.
The OS: Really that's up to you, you'll want Windows for sure but then rises the question: 7 or 8? I personally have gone with 8 and I like it a lot, it just needs some getting used to. If you don't like it or just want to use something you already are comfortable with, go for 7. I think prices for the OSes lay around 100 euro's.
The Case: Again, this is about personal preference although if you want a decent one you'll quickly be looking at around 100 euro's. Mine is the Cooler Master Centurion 6 which is good especially price wise, I mean I've only spent 60 euro's on it so I really can't complain about it
The CPU: Here I would rather stick with Intel, because AMD only really makes sense when you don't have a dedicated graphics card (which if you are going to game/photoshop you will need). For both systems you'll want to get yourself an i5, I can really recommend the 4670K (or non-the K version if you're not planning on overclocking, that will save you about 20 euro's more but can't be overclocked = no extra performance from that), they are great value for money and for gaming there really isn't a reason why you'd want an i7, it won't give you any increased performance (the only real difference (except for increased clock speed maybe, which doesn't really matter if you overclock) is that the i7 has hyperthreading, this means that every physical core is split up in two 'virtual' cores. This allows for better performance if you run a lot of tasks next to each other or do video editing and heavy 3d rendering, but neither gaming or photoshop really benefit form it so it isn't worth the extra cost of about 100 euro's). You can find the i5-4670K for about 200 euro's.
CPU Cooler: With haswell, you want a decent cooler and especially if you're overclocking. Good air coolers are the Scythe Mugen Series 4 but I'm not really into air cooling
those will set you back for about 40 euro's. If you want better cooling, I highly suggest you get yourself one of the Corsair Hydro Series with an i at the end of the name, they are updated to deliver better cooling performance than the non i version and are more quite. Prices vary form 60 (H60 (there is no i here but actually there should be, it has gotten the same updates as the i series)) to 100 euro's (H100i, I have this and it is really good, it keeps my 4670K@4.5GHz on 1.31vcore fairly cool). The next thing you'll want is thermal paste, the corsair ones come with it pre-applied but just get yourself a bit just to be sure. The expensive ones might get you 1 or 2 degrees lower but you can find thermal paste for around 7 euro's.
The Motherboard: According to the case you selected you must first determine the size, but most likely it's going to be ATX (standard size). Next thing is the socket, if you have a haswell processor (the i5-4670(k) is one) then you want the 1550 socket, but this is always noted with the processor specs. You'll also want to get yourself the Z chipset if you have a K processor, this is the best chipset if you want to overclock or just for gaming overall. If you want to save money here you can always get a H chipset but I don't recommend it for a gaming system. A decent motherboard will set you back for about 150 euro's. I personally own an MSI Z87-G45 Gaming which is amazing costs 130 euro's and can compete very good with boards that are 50 euro's more expensive (it is the same as the 50 euro's more expensive GD65 with the only difference that it has 2 less sata ports, but it still has 6 which is more than enough normally
)
The RAM : The standard of today is DDR3-1600MHz, so you should get that. No need in buying 1866MHz or higher, you won't really notice much difference, it's mostly for benchmarks (you can always overclock your memory as well, I have my 16gigs of Corsair Vengeance DDR3-1600 running at 2133MHz). You should be good with 8GB of RAM, though 16 is also an option. I wouldn't go further than that since it won't really help you (unless you run like 20 heavy programs at the same time, but I assume you don't
). Another thing that you should look after is CAS latency (the CL*x* part in the model name, CL9 is most common I think but just remember that lower CL = better). A good pair of 8GB will set you back for about 70 euro's.
The GPU: As I am kind of an nVidia fan myself, I won't be talking about AMD cards here. For your setups you will want to grab the latest series available (700 series). I highly recommend the GTX760 2GB Gaming from MSI, it comes factory overclocked and has really good cooling, but you can choose other brands if you want. The 760 is probably the best option considering your budget, it will run BF4 maxed out @1080p without much problems (+-60fps with the occasional drop to 40). About 230 euro's. You might also want to take a look at the newly released GTX750 (or the ti version, a bit more expensive but better), they are not so much behind the 760 and come for about 150 euro's. These cards can all be overclocked for better performance
The storage: If you can afford to spend a bit more, an SSD would be the way to go. It is much faster than a normal HDD but it comes at a cost. If you want a good one, just for Windows and your programs, get the Samsung EVO 120GB, it comes at around 80 euro's and is really fast (Samsung has currently the fastest SSD's out there, while the EVO is not the first-class series, it still is blazing fast). For a normal hard drive, you can get one of those really cheaply, 2TB for about 60 euro's. Just make sure it sais SATA3 and 7200RPM. You might also want a DVD burner, +-20 euro's (Blu-ray burners come at +-50 euro's)
The PSU: A good website to calculate how much your system will use is this one
http://, it will also display a good PSU for the calculated wattage. Normally Corsair, Seasonic, Antec, XFX, OCZ, Cooler Master are good brands for PSU's. You should find one for about 100 euro's.
End price if every component is the best I listed: +-1250 euro's, so you might want to adjust the selection a bit to see what fits you best
Well, this wraps it about up I think
Hope I helped you making the right decisions
#EDIT: I noticed someone else already commented during the time I was writing this, as you can see much of our recommendations are similar, still hope I helped though 
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